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+/* gzjoin -- command to join gzip files into one gzip file
+
+  Copyright (C) 2004, 2005, 2012 Mark Adler, all rights reserved
+  version 1.2, 14 Aug 2012
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the author be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+
+  Mark Adler    madler@alumni.caltech.edu
+ */
+
+/*
+ * Change history:
+ *
+ * 1.0  11 Dec 2004     - First version
+ * 1.1  12 Jun 2005     - Changed ssize_t to long for portability
+ * 1.2  14 Aug 2012     - Clean up for z_const usage
+ */
+
+/*
+   gzjoin takes one or more gzip files on the command line and writes out a
+   single gzip file that will uncompress to the concatenation of the
+   uncompressed data from the individual gzip files.  gzjoin does this without
+   having to recompress any of the data and without having to calculate a new
+   crc32 for the concatenated uncompressed data.  gzjoin does however have to
+   decompress all of the input data in order to find the bits in the compressed
+   data that need to be modified to concatenate the streams.
+
+   gzjoin does not do an integrity check on the input gzip files other than
+   checking the gzip header and decompressing the compressed data.  They are
+   otherwise assumed to be complete and correct.
+
+   Each joint between gzip files removes at least 18 bytes of previous trailer
+   and subsequent header, and inserts an average of about three bytes to the
+   compressed data in order to connect the streams.  The output gzip file
+   has a minimal ten-byte gzip header with no file name or modification time.
+
+   This program was written to illustrate the use of the Z_BLOCK option of
+   inflate() and the crc32_combine() function.  gzjoin will not compile with
+   versions of zlib earlier than 1.2.3.
+ */
+
+#include <stdio.h>      /* fputs(), fprintf(), fwrite(), putc() */
+#include <stdlib.h>     /* exit(), malloc(), free() */
+#include <fcntl.h>      /* open() */
+#include <unistd.h>     /* close(), read(), lseek() */
+#include "zlib.h"
+    /* crc32(), crc32_combine(), inflateInit2(), inflate(), inflateEnd() */
+
+#define local static
+
+/* exit with an error (return a value to allow use in an expression) */
+local int bail(char *why1, char *why2)
+{
+    fprintf(stderr, "gzjoin error: %s%s, output incomplete\n", why1, why2);
+    exit(1);
+    return 0;
+}
+
+/* -- simple buffered file input with access to the buffer -- */
+
+#define CHUNK 32768         /* must be a power of two and fit in unsigned */
+
+/* bin buffered input file type */
+typedef struct {
+    char *name;             /* name of file for error messages */
+    int fd;                 /* file descriptor */
+    unsigned left;          /* bytes remaining at next */
+    unsigned char *next;    /* next byte to read */
+    unsigned char *buf;     /* allocated buffer of length CHUNK */
+} bin;
+
+/* close a buffered file and free allocated memory */
+local void bclose(bin *in)
+{
+    if (in != NULL) {
+        if (in->fd != -1)
+            close(in->fd);
+        if (in->buf != NULL)
+            free(in->buf);
+        free(in);
+    }
+}
+
+/* open a buffered file for input, return a pointer to type bin, or NULL on
+   failure */
+local bin *bopen(char *name)
+{
+    bin *in;
+
+    in = malloc(sizeof(bin));
+    if (in == NULL)
+        return NULL;
+    in->buf = malloc(CHUNK);
+    in->fd = open(name, O_RDONLY, 0);
+    if (in->buf == NULL || in->fd == -1) {
+        bclose(in);
+        return NULL;
+    }
+    in->left = 0;
+    in->next = in->buf;
+    in->name = name;
+    return in;
+}
+
+/* load buffer from file, return -1 on read error, 0 or 1 on success, with
+   1 indicating that end-of-file was reached */
+local int bload(bin *in)
+{
+    long len;
+
+    if (in == NULL)
+        return -1;
+    if (in->left != 0)
+        return 0;
+    in->next = in->buf;
+    do {
+        len = (long)read(in->fd, in->buf + in->left, CHUNK - in->left);
+        if (len < 0)
+            return -1;
+        in->left += (unsigned)len;
+    } while (len != 0 && in->left < CHUNK);
+    return len == 0 ? 1 : 0;
+}
+
+/* get a byte from the file, bail if end of file */
+#define bget(in) (in->left ? 0 : bload(in), \
+                  in->left ? (in->left--, *(in->next)++) : \
+                    bail("unexpected end of file on ", in->name))
+
+/* get a four-byte little-endian unsigned integer from file */
+local unsigned long bget4(bin *in)
+{
+    unsigned long val;
+
+    val = bget(in);
+    val += (unsigned long)(bget(in)) << 8;
+    val += (unsigned long)(bget(in)) << 16;
+    val += (unsigned long)(bget(in)) << 24;
+    return val;
+}
+
+/* skip bytes in file */
+local void bskip(bin *in, unsigned skip)
+{
+    /* check pointer */
+    if (in == NULL)
+        return;
+
+    /* easy case -- skip bytes in buffer */
+    if (skip <= in->left) {
+        in->left -= skip;
+        in->next += skip;
+        return;
+    }
+
+    /* skip what's in buffer, discard buffer contents */
+    skip -= in->left;
+    in->left = 0;
+
+    /* seek past multiples of CHUNK bytes */
+    if (skip > CHUNK) {
+        unsigned left;
+
+        left = skip & (CHUNK - 1);
+        if (left == 0) {
+            /* exact number of chunks: seek all the way minus one byte to check
+               for end-of-file with a read */
+            lseek(in->fd, skip - 1, SEEK_CUR);
+            if (read(in->fd, in->buf, 1) != 1)
+                bail("unexpected end of file on ", in->name);
+            return;
+        }
+
+        /* skip the integral chunks, update skip with remainder */
+        lseek(in->fd, skip - left, SEEK_CUR);
+        skip = left;
+    }
+
+    /* read more input and skip remainder */
+    bload(in);
+    if (skip > in->left)
+        bail("unexpected end of file on ", in->name);
+    in->left -= skip;
+    in->next += skip;
+}
+
+/* -- end of buffered input functions -- */
+
+/* skip the gzip header from file in */
+local void gzhead(bin *in)
+{
+    int flags;
+
+    /* verify gzip magic header and compression method */
+    if (bget(in) != 0x1f || bget(in) != 0x8b || bget(in) != 8)
+        bail(in->name, " is not a valid gzip file");
+
+    /* get and verify flags */
+    flags = bget(in);
+    if ((flags & 0xe0) != 0)
+        bail("unknown reserved bits set in ", in->name);
+
+    /* skip modification time, extra flags, and os */
+    bskip(in, 6);
+
+    /* skip extra field if present */
+    if (flags & 4) {
+        unsigned len;
+
+        len = bget(in);
+        len += (unsigned)(bget(in)) << 8;
+        bskip(in, len);
+    }
+
+    /* skip file name if present */
+    if (flags & 8)
+        while (bget(in) != 0)
+            ;
+
+    /* skip comment if present */
+    if (flags & 16)
+        while (bget(in) != 0)
+            ;
+
+    /* skip header crc if present */
+    if (flags & 2)
+        bskip(in, 2);
+}
+
+/* write a four-byte little-endian unsigned integer to out */
+local void put4(unsigned long val, FILE *out)
+{
+    putc(val & 0xff, out);
+    putc((val >> 8) & 0xff, out);
+    putc((val >> 16) & 0xff, out);
+    putc((val >> 24) & 0xff, out);
+}
+
+/* Load up zlib stream from buffered input, bail if end of file */
+local void zpull(z_streamp strm, bin *in)
+{
+    if (in->left == 0)
+        bload(in);
+    if (in->left == 0)
+        bail("unexpected end of file on ", in->name);
+    strm->avail_in = in->left;
+    strm->next_in = in->next;
+}
+
+/* Write header for gzip file to out and initialize trailer. */
+local void gzinit(unsigned long *crc, unsigned long *tot, FILE *out)
+{
+    fwrite("\x1f\x8b\x08\0\0\0\0\0\0\xff", 1, 10, out);
+    *crc = crc32(0L, Z_NULL, 0);
+    *tot = 0;
+}
+
+/* Copy the compressed data from name, zeroing the last block bit of the last
+   block if clr is true, and adding empty blocks as needed to get to a byte
+   boundary.  If clr is false, then the last block becomes the last block of
+   the output, and the gzip trailer is written.  crc and tot maintains the
+   crc and length (modulo 2^32) of the output for the trailer.  The resulting
+   gzip file is written to out.  gzinit() must be called before the first call
+   of gzcopy() to write the gzip header and to initialize crc and tot. */
+local void gzcopy(char *name, int clr, unsigned long *crc, unsigned long *tot,
+                  FILE *out)
+{
+    int ret;                /* return value from zlib functions */
+    int pos;                /* where the "last block" bit is in byte */
+    int last;               /* true if processing the last block */
+    bin *in;                /* buffered input file */
+    unsigned char *start;   /* start of compressed data in buffer */
+    unsigned char *junk;    /* buffer for uncompressed data -- discarded */
+    z_off_t len;            /* length of uncompressed data (support > 4 GB) */
+    z_stream strm;          /* zlib inflate stream */
+
+    /* open gzip file and skip header */
+    in = bopen(name);
+    if (in == NULL)
+        bail("could not open ", name);
+    gzhead(in);
+
+    /* allocate buffer for uncompressed data and initialize raw inflate
+       stream */
+    junk = malloc(CHUNK);
+    strm.zalloc = Z_NULL;
+    strm.zfree = Z_NULL;
+    strm.opaque = Z_NULL;
+    strm.avail_in = 0;
+    strm.next_in = Z_NULL;
+    ret = inflateInit2(&strm, -15);
+    if (junk == NULL || ret != Z_OK)
+        bail("out of memory", "");
+
+    /* inflate and copy compressed data, clear last-block bit if requested */
+    len = 0;
+    zpull(&strm, in);
+    start = in->next;
+    last = start[0] & 1;
+    if (last && clr)
+        start[0] &= ~1;
+    strm.avail_out = 0;
+    for (;;) {
+        /* if input used and output done, write used input and get more */
+        if (strm.avail_in == 0 && strm.avail_out != 0) {
+            fwrite(start, 1, strm.next_in - start, out);
+            start = in->buf;
+            in->left = 0;
+            zpull(&strm, in);
+        }
+
+        /* decompress -- return early when end-of-block reached */
+        strm.avail_out = CHUNK;
+        strm.next_out = junk;
+        ret = inflate(&strm, Z_BLOCK);
+        switch (ret) {
+        case Z_MEM_ERROR:
+            bail("out of memory", "");
+        case Z_DATA_ERROR:
+            bail("invalid compressed data in ", in->name);
+        }
+
+        /* update length of uncompressed data */
+        len += CHUNK - strm.avail_out;
+
+        /* check for block boundary (only get this when block copied out) */
+        if (strm.data_type & 128) {
+            /* if that was the last block, then done */
+            if (last)
+                break;
+
+            /* number of unused bits in last byte */
+            pos = strm.data_type & 7;
+
+            /* find the next last-block bit */
+            if (pos != 0) {
+                /* next last-block bit is in last used byte */
+                pos = 0x100 >> pos;
+                last = strm.next_in[-1] & pos;
+                if (last && clr)
+                    in->buf[strm.next_in - in->buf - 1] &= ~pos;
+            }
+            else {
+                /* next last-block bit is in next unused byte */
+                if (strm.avail_in == 0) {
+                    /* don't have that byte yet -- get it */
+                    fwrite(start, 1, strm.next_in - start, out);
+                    start = in->buf;
+                    in->left = 0;
+                    zpull(&strm, in);
+                }
+                last = strm.next_in[0] & 1;
+                if (last && clr)
+                    in->buf[strm.next_in - in->buf] &= ~1;
+            }
+        }
+    }
+
+    /* update buffer with unused input */
+    in->left = strm.avail_in;
+    in->next = in->buf + (strm.next_in - in->buf);
+
+    /* copy used input, write empty blocks to get to byte boundary */
+    pos = strm.data_type & 7;
+    fwrite(start, 1, in->next - start - 1, out);
+    last = in->next[-1];
+    if (pos == 0 || !clr)
+        /* already at byte boundary, or last file: write last byte */
+        putc(last, out);
+    else {
+        /* append empty blocks to last byte */
+        last &= ((0x100 >> pos) - 1);       /* assure unused bits are zero */
+        if (pos & 1) {
+            /* odd -- append an empty stored block */
+            putc(last, out);
+            if (pos == 1)
+                putc(0, out);               /* two more bits in block header */
+            fwrite("\0\0\xff\xff", 1, 4, out);
+        }
+        else {
+            /* even -- append 1, 2, or 3 empty fixed blocks */
+            switch (pos) {
+            case 6:
+                putc(last | 8, out);
+                last = 0;
+            case 4:
+                putc(last | 0x20, out);
+                last = 0;
+            case 2:
+                putc(last | 0x80, out);
+                putc(0, out);
+            }
+        }
+    }
+
+    /* update crc and tot */
+    *crc = crc32_combine(*crc, bget4(in), len);
+    *tot += (unsigned long)len;
+
+    /* clean up */
+    inflateEnd(&strm);
+    free(junk);
+    bclose(in);
+
+    /* write trailer if this is the last gzip file */
+    if (!clr) {
+        put4(*crc, out);
+        put4(*tot, out);
+    }
+}
+
+/* join the gzip files on the command line, write result to stdout */
+int main(int argc, char **argv)
+{
+    unsigned long crc, tot;     /* running crc and total uncompressed length */
+
+    /* skip command name */
+    argc--;
+    argv++;
+
+    /* show usage if no arguments */
+    if (argc == 0) {
+        fputs("gzjoin usage: gzjoin f1.gz [f2.gz [f3.gz ...]] > fjoin.gz\n",
+              stderr);
+        return 0;
+    }
+
+    /* join gzip files on command line and write to stdout */
+    gzinit(&crc, &tot, stdout);
+    while (argc--)
+        gzcopy(*argv++, argc, &crc, &tot, stdout);
+
+    /* done */
+    return 0;
+}